moc: Allow reading property values through bindables

The behavior is similar to MEMBER: If the READ accessor is "default",
synthesize it using the bindable.

[ChangeLog][QtCore] You can now specify "READ default" in a Q_PROPERTY
if you also specify a BINDABLE. moc will synthesize a READ accessor in
that case.

Task-number: QTBUG-97249
Change-Id: I4a275adabaed12df95dac505095f847c4be65dfe
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/doc/src/objectmodel/properties.qdoc

@@ -44,7 +44,9 @@
     specified. It is for reading the property value. Ideally, a const function
     is used for this purpose, and it must return either the property's type or a
     const reference to that type. e.g., QWidget::focus is a read-only
-    property with \c READ function, QWidget::hasFocus().
+    property with \c READ function, QWidget::hasFocus(). If a \c BINDABLE is
+    specified, you can write \c{READ default} to have the \c READ accessor
+    generated from the \c BINDABLE.
 
     \li A \c WRITE accessor function is optional. It is for setting the
     property value. It must return void and must take exactly one

src/tools/moc/generator.cpp

@@ -1297,6 +1297,9 @@ void Generator::generateStaticMetacall()
                 else if (cdef->enumDeclarations.value(p.type, false))
                     fprintf(out, "        case %d: *reinterpret_cast<int*>(_v) = QFlag(%s%s()); break;\n",
                             propindex, prefix.constData(), p.read.constData());
+                else if (p.read == "default")
+                    fprintf(out, "        case %d: *reinterpret_cast< %s*>(_v) = %s%s().value(); break;\n",
+                            propindex, p.type.constData(), prefix.constData(), p.bind.constData());
                 else if (!p.read.isEmpty())
                     fprintf(out, "        case %d: *reinterpret_cast< %s*>(_v) = %s%s(); break;\n",
                             propindex, p.type.constData(), prefix.constData(), p.read.constData());

src/tools/moc/moc.cpp

@@ -1306,6 +1306,10 @@ void Moc::parsePropertyAttributes(PropertyDef &propDef)
             v = lexem();
             if (l != "REVISION")
                 error(1);
+        } else if (test(DEFAULT)) {
+            v = lexem();
+            if (l != "READ")
+                error(1);
         } else {
             next(IDENTIFIER);
             v = lexem();
@@ -1384,6 +1388,12 @@ void Moc::parsePropertyAttributes(PropertyDef &propDef)
         propDef.constant = false;
         warning(msg.constData());
     }
+    if (propDef.read == "default" && propDef.bind.isNull()) {
+        const QByteArray msg = "Property declaration " + propDef.name
+                + " is not BINDable but default-READable. READ will be ignored.";
+        propDef.read = "";
+        warning(msg.constData());
+    }
 }
 
 void Moc::parseProperty(ClassDef *def)

tests/auto/tools/moc/tst_moc.cpp

@@ -743,6 +743,7 @@ private slots:
     void observerMetaCall();
     void setQPRopertyBinding();
     void privateQPropertyShim();
+    void readThroughBindable();
 
 signals:
     void sigWithUnsignedArg(unsigned foo);
@@ -4329,6 +4330,44 @@ void tst_Moc::privateQPropertyShim()
     QCOMPARE(testObject.priv.testProperty2.value(), 42);
 }
 
+
+class BindableOnly : public QObject
+{
+    Q_OBJECT
+    Q_PROPERTY(int score BINDABLE scoreBindable READ default)
+public:
+    BindableOnly(QObject *parent = nullptr)
+        : QObject(parent)
+        , m_score(4)
+    {}
+    QBindable<int> scoreBindable() { return QBindable<int>(&m_score); }
+private:
+    QProperty<int> m_score;
+};
+
+
+void tst_Moc::readThroughBindable()
+{
+    BindableOnly o;
+
+    QCOMPARE(o.scoreBindable().value(), 4);
+    QCOMPARE(o.property("score").toInt(), 4);
+    o.scoreBindable().setValue(5);
+    QCOMPARE(o.scoreBindable().value(), 5);
+    QCOMPARE(o.property("score").toInt(), 5);
+
+
+    const QMetaObject *mo = o.metaObject();
+    const int i = mo->indexOfProperty("score");
+    QVERIFY(i > 0);
+
+    QMetaProperty p = mo->property(i);
+    QCOMPARE(p.name(), "score");
+
+    QVERIFY(p.isValid());
+    QCOMPARE(p.read(&o), 5);
+}
+
 QTEST_MAIN(tst_Moc)
 
 // the generated code must compile with QT_NO_KEYWORDS